1. Field of the Invention
The present invention relates to heat sources, and particularly to a nonconsumable electrode for melting metals and alloys. Most advantageously the invention can be used in electrometallurgical installations for melting metals and alloys, in appratuses for the partial melting and building-up of ingots as well as for producing metals and alloys from lump charge in vacuum furnaces with a water-cooled mould or skull crucibles and in arc furnaces with a ceramic lining.
2. Description of the Prior Art
To provide a nonconsumable electrode which would be reliable in service and have a long service life is a long-standing problem. However, efforts made to solve this problem have not yielded satisfactory results so far.
The prior art consumable electrodes have a common principle of operation based on moving the arc spot around the arcing surface of the electrode tip. While moving over the arcing surface, the arc heats different places thereof, thereby causing its deformation, which decreases the length of the electrode life. The currently used nonconsumable electrodes reliably operate only from 40 to 100 hours and therefore are not capable of adequately satisfying requirements of the electrometallurgy, taking into account that the production of one large ingot takes 5-10 hours. Thus, the tip of the above nonconsumable electrode is capable of providing only a limited number of heats. Frequent replacement of costly tips of the nonconsumable electrodes brings down the efficiency of the melting process and raises production costs.
There are known nonconsumable electrodes for melting metals and alloys (cf. U.S. Pat. No. 3,649,733 and No. 3,651,239) comprising a hollow body to one end of which is secured a detachable hollow tip having an arcing surface, and wherein there is provided a partition mounted within the cavity of the body and of the tip so as to form between said partition and the walls of the body and of the tip a passageway for the flow of a coolant. These electrodes are installed in furnaces generally at a certain angle relative to the metal bath. In operation the electrode is rotated about its axis with about 250 r.p.m. While the electrode is rotated, the intensely hot spot of the electric arc established between the tip arcing surface and the melt moves over the arcing surface of the cooled tip, which accounts for its insignificant wear. Melting is effected with the aid of an electric arc of opposite polarity in which case the positive pole is connected to the nonconsumable electrode. Such a construction of a nonconsumable electrode permits production of metals and alloys from lump charge, managing without consumable electrodes. However, the presence of movable electric conductors and cooling-water supply conduits complicates the construction of this nonconsumable electrode. In addition, special drives are required for rotating such a nonconsumable electrode, which considerably complicates the melting apparatus as a whole. Furthermore, the complexity of vacuum sealing while lowering the electrode into a furance limits to a great extent the speed of its rotation, which in turn leads to using electrodes with a larger diameter (from 0.5 m and larger). All these disadvantages impair the reliability of these electrodes and bring down their thermal efficiency.
There are also known nonconsumable electrodes for melting metals and alloys (cf. U.S. Pat. No. 3,610,796 and British Pat. No. 1,325,522), comprising a hollow body with a hollow detachable electrode tip secured to one of the ends thereof and having an arcing surface, and a partition mounted in the cavity of the body and the tip so that between said partition and the walls of the body and of the tip a passageway is formed for the flow of a coolant. Disposed within the tip are permanent magnets. In the course of melting a powerful arc is established between the sparking surface of the tip and the metal being melted. The magnetic poles of the magnets are arranged in the cavity of the tip so as to cause the arc and hence the arc spot to move around the arcing surface, in which case the erosion of the arcing surface is minimized. Melting metal is generally effected by an arc having opposite polarity, which provides for a satisfactory durability of the electrode tip in the case of low velocity of the arc movement. However, the operational reliability of this nonconsumable electrode is achieved by using powerful permanent magnets, which results in the increase of the dimensions of the electrode and hence brings down its thermal efficiency. Moreover, the electrode tip has a special configuration, which complicates its manufacture and thereby raises its cost.
Known in the art are also nonconsumable electrodes for melting metals and alloys (cf. U.S. Pat. No. 3,368,018, No. 3,369,067 and No. 3,480,717), comprising a hollow body with a hollow detachable electrode tip secured to one of the ends thereof and having an arcing surface, and a partition mounted in the cavity of the body and the tip so that between said partition and the walls of the body and of the tip a passageway is formed for the coolant to pass therethrough, and wherein a solenoid is provided to set up a magnetic field for moving the electric arc ignited between the arcing surface and the metal to be melted, said solenoid being placed in a special housing made of magnetic material and disposed within the electrode tip. The electrode tip and the solenoid are fed from separate d.c. sources. In order to decrease the probability of break-down of the solenoid insulation said solenoid is fed from a low-voltage d.c. source. In the course of melting the solenoid produces a strong magnetic field which is superposed with the magnetic field of the arc, thereby causing the latter to move circularly on the arcing surface of the electrode tip. The necessary of setting-up of a strong radial magnetic field parallel to the arcing surface of the tip accounts for that the lower end of the solenoid is located as near as possible to the arcing surface. Moving the arc spot over the arcing surface of the tip provides for a high resistance thereof to erosion even at elevated temperatures at the intensely hot arc spot, ranging from 4 to 5.10.sup.10 kcal/m.sup.2 per hour. The velocity of the arc movement is controlled by varying magnitude of the direct current feeding the solenoid. Melting metals with the aid of such nonconsumable electrode can be effected by an arc either of direct or opposite polarity. However, melting metals by the arc of direct polarity requires that the arc be moved with the velocity higher than 1000 m/sec., which can be achieved by setting-up of a strong magnetic field which, in turn, requires a current of a great magnitude for feeding the solenoid. Employing special sources of low voltage strong direct current for feeding solenoid requires additional space for the disposal thereof, raises the cost of the nonconsumable electrode, as well as complicates controlling the melting process and brings down the operational reliability of the electrode.
The above disadvantages have been overcome in the non-consumable electrode for melting metals and alloys (cf. U.S. Pat. No. 4,004,076) comprising a hollow cylindrical body with a hollow detachable electrode tip secured to one of the ends thereof, a solenoid intended for generating a magnetic field and being at least partially disposed within the electrode tip, and a partition coaxially mounted within the cylindrical body and the electrode tip so as to form between said partition and the walls of the body and of the tip a passageway for a coolant to pass therethrough. The hollow tip is formed as a body of revolution coaxial with the cylindrical body and has a lateral cylindrical non-arcing surface and a toroidal arcing and surface. The solenoid turns are made in the form of helical projections on the lateral wall of the hollow tip and connected in series to a d.c. source. The lower end surface of the solenoid is disposed in proximity to the arcing surface of the tip in order to set up a high radial magnetic field parallel to the arcing surface thereof. When the arc is ignited between the arcing surface to the tip and a metal to be melt, the current from a common source flows through the projections formed by the helical groove on the lateral wall of the tip and through the lateral wall thereof, thus generating a magnetic field. Since the electric arc generates its own magnetic field, the magnetic field generated by the helical projection on the lateral wall of the tip, being superposed with the arc magnetic field, causes the arc to move along a circular path on the arcing surface with a predetermined velocity. The solenoid turns being seriesly connected in the arc circuit rules out the necessity of using additional d.c. sources, which simplifies the control over the melting process and enhances its reliability. In addition, a nonconsumable electrode wherein the solenoid turns are helical projections formed on the lateral wall of the tip cooled by a coolant, has a simple construction and a long useful life. However, the arc hot spot moving over the toroidal arcing surface of the tip produces varying thermal fields in the material thereof, whereby causing deformation of the tip surface, which considerably decreases the service life of the electrode tip. Furthermore, a rotating high-power electric arc renders electrical parameters unstable. At the same time, a toroidal arcing surface necessitates the presence of an axial cavity the diameter of which should be not less than the diameter of the arc hot spot, which is necessary for providing a stable circular arc movement. Otherwise, the arc may change from the circular to a straightline path along the diameter of the tip thereby, causing destruction thereof. With the increase of the axial cavity diameter the diameter of the whole nonconsumable electrode also increases, which brings down a thermal efficiency of the furnace as a result of thermal losses. In operation, on the walls of the axial cavity of the tip arcing surface there are condensed vapors and spatters of molten metal, which changes the configuration of the arcing surface and affects the stability of the arc movement. On the other hand in order to set up a high radial magnetic field parallel to the arcing surface of the tip, the lower end surface of the solenoid should be disposed as near to the arcing surface of the tip as possible. For cooling the tip, and especially the arcing surface thereof the passageways of a certain cross-section have to be formed to enable a coolant to pass between the lower end surface of the solenoid and the arcing surface of the tip. With the increase of the cross-section of the passageways the distance between the lower end surface of the solenoid and the arcing surface decreases thereby lowering the magnetic field intensity, whereas decreasing the cross-section of these passageways leads to the necessity of increasing the pressure in the coolant supply system which in the case of melting highly reactive metals may cause destruction of the walls of the tip.